1. Field of the Invention
The present invention relates to a positioning system, a positioning IC chip, a positioning method, and a positioning program for estimating the position of a receiver.
2. Description of the Related Art
Conventionally, a technology which performs positioning of a receiver by using the distances from satellites (including pseudo-satellites) which are positioning signal sources to the receiver such as GPS (Global Positioning System) and Galileo and so forth are known. There are also positioning systems known as hybrids which perform positioning by combining the usage of a signal from the satellite and a signal of the mobile communication network.
With the above positioning method, a positioning calculation is performed by calculating the distances from the satellites to the receiver on the basis of the transmission times and reception times of the positioning signals transmitted by the satellites and received by the receiver and using the distances thus calculated. If the distances from a plurality of satellites to the receiver are determined exactly, the position of the receiver can be calculated by finding the intersection between spheres at whose center the satellites are located and whose radii are the determined distances. However, it is difficult to calculate the exact distances because a variety of elements capable of producing an error exist in the distance calculation.
Therefore, the position of the receiver is calculated using a method such as the least-square method from the calculated distances between the plurality of satellites and the receiver. Here, because the error in the calculated distance between the satellite and the receiver differs for each satellite, there is sometimes a difference between the calculated position of the receiver and the actual position of the receiver as mentioned earlier. That is, a positioning error normally occurs with the above method.
Such a positioning error is evaluated, that is, the positioning accuracy is evaluated. The evaluated positioning accuracy is used in the judgment to provide the user with positioning information in cases where the positioning accuracy is high, for example. Positioning accuracy evaluation methods conventionally include the method known as RAIM (Receiver Autonomous Integrity Monitoring). With RAIM, the combination of satellites used for the positioning is changed and an evaluation of the positioning accuracy is made on the basis of changes in the positioning results between these combinations of satellites. Details of the above appear in B W Parkinson, J J Spilker, “Global Positioning System: Theory and Applications Volume II-Chapter 5”, American Institute of Aeronautics and Astronautics, Inc., pages 143 to 164, 1996 (Non-Patent Document 1).
However, because a minimum of five positioning satellites are required with RAIM, there has been the problem that the receiver needs to capture a large number of satellites. In addition, with RAIM, the positioning calculation using the combination of satellites must be repeated five times or more.